Britain could withdraw its troops from Afghanistan faster than expected next year, as military commanders reassess how many troops they need to help local forces fight the Taliban, Britain's defence secretary said in an interview.

Britain is due to withdraw 500 soldiers by the end of this year, leaving 9,000 soldiers to help Afghan National Security Forces, but Philip Hammond said there was potential to further "draw down troops in 2013."

"Six months ago the message coming from them [the military] was that we really need to hold on to everything we have got for as long as we possibly can. I think they are seeing potentially more flexibility in the situation," Hammond told Friday's Guardian newspaper.

He did not go into details and said no decisions had been taken on an accelerated withdrawal of troops, with the bulk of Britain's soldiers planned to return home by the end of 2014.

"I think there is a bit of a rethinking going on about how many troops we do actually need," said Hammond.

"There may be some scope for a little bit more flexibility on the way we draw down, and that is something commanders on the ground are looking at very actively."

He said British military thinking was evolving because commanders had been "surprised by the extent to which they have been able to draw back and leave the Afghans to take the lion's share of the combat role".

Hammond said the security situation was improving in Helmand province in Afghanistan's southwest, where British troops have mainly been based, after some of the bloodiest fighting in the Afghan conflict and the deaths of more than 20 British troops in recent months.